A method of forming a conductor circuit such as a printed circuit board is known in the art, which comprises coating a photocurable resist composition onto a board, followed by exposing to light, developing to form a resist pattern, and by etching to remove unnecessary portion.
For example, a photocurable resist composition capable of being developed with a weak alkali by use of an unsaturated resin having carboxyl group is known as the above photocurable resist composition (see Japanese Patent Application Laid-Open No. 223759/91).
The above carboxyl group-containing unsaturated resin is usually prepared as described in the above prior art by a process which comprises subjecting an acid unsaturated monomer such as acrylic acid with alkyl (meth)acrylate monomer to a radical copolymerization reaction to obtain a polycarboxylic acid resin, followed by subjecting the resin and an epoxy group-containing unsaturated monomer such as glycidyl (meth)acrylate to an addition reaction of a part of the carboxyl group-containing alkyl (meth)acrylate monomer with glycidyl group.
The use of the above acid resin as an alkali development type resist composition had such problems (1) that a broad molecular weight distribution of the acid resin prepared by the radical polymerization reaction causes a reduction of solubility due to an alkali developing solution or etching solution in a high molecular weight region and an increase of solubility due to the alkali developing solution or etching solution in a low molecular weight region, resulting in making it impossible to carry out a uniform developing treatment or etching treatment, (2) that difference of a radical copolymerization reaction speed between a (meth)acrylic acid monomer component and alkyl (meth)acrylate monomer may produce a homopolymer of the acrylic acid monomer or an acrylic resin containing the acrylic acid component in a small amount, resulting in that formation of a fine resist pattern is made impossible due to non-uniform speed of removing the resist film by the alkali developing treatment, remaining of the resist film in a short period of treating time, and to erosion and washing out of a photocured film, (3) that heating on the addition reaction between the polycarboxylic acid resin and the epoxy group-containing unsaturated monomer further increases molecular weight of the polycarboxylic acid resin, (4) that unnecessary presence with the resin of a radical polymerization inhibitor usually added for the purpose of inhibiting the radical polymerization reaction unsaturated groups on the addition reaction reduces reactivity of the photopolymerization reaction, and (5) that erosion and washing out of the photocured film by the etching solution due to unsatisfactory properties of the photocured film makes it impossible to form a fine resist pattern.
As a method of introducing an unsaturated group into the resin, in addition to the above method, for example, Japanese Patent Application Laid-Open No. 102037/95 discloses a process for preparing a water-soluble actinic radiation-curable resin which comprises reacting a polyhydroxy compound, radically polymerizable unsaturated group-containing polyhydroxy compound, anionic hydrophilic group-containing polyhydroxy compound, polyisocyanate compound and radically polymerizable unsaturated group-containing monohydroxy compound to obtain a polyurethane resin, followed by neutralizing with an amine. However, use of the above water-soluble polyurethane resin as a resist composition produced such problems that poor alkali developing properties and anti-etching properties make it impossible to form a fine resist pattern.
As another method of introducing an unsaturated group into the resin, Japanese Patent Application Laid-Open No. 136077/94 discloses a radiation-curable resin composition prepared by reacting a reaction product of dimethylol propionic acid with .epsilon.-caprolactone, organic polyisocyanate compound and hydroxyl group-containing (meth)acrylate. However, use of the above resin composition as the resist composition had problems of poor properties in alkali developing properties, anti-ethcing properties and the like.